Although fiberglass and fluorocarbon material, such as polytetrafluoroethylene sold under the trademark TEFLON by the Dupont Company, have respectively been used as high temperature resistant insulation about electrical conductors for many years, no one heretofore had thought to provide the combination of fiberglass and electrostatically deposited fluorocarbon material as a high temperature resistant insulation about an electrical conductor.
Although fiberglass insulation is characteristically applied by spiraling or by braiding fiberglass filaments or strands about the conductor or a combination of the two, fluorocarbon materials have not heretofore been known to have been applied to an electrical conductor (wire) by electrostatic deposition let alone onto fiberglass encompassing the conductor.
The method by which fluorocarbon materials (polymers) have heretofore been applied to insulate an electrical conductor (wire) have been wrought with problems.
The fluorocarbon material polytetrafluoroethylene (PTFE) for example has excellent high temperature resistance (500 .degree. F.) but must be applied to electrical conductor by ram extrusion or by wrapping skived PTFE tape about the conductor. One of the problems with ram extrusion is that a slug or circular billet of PTFE being ram extruded through an orifice into a tubular configuration is generally made from a mixture of PTFE powder and solvents and the coated conductor (wire) must then be heated (sintered) to drive off solvent volatiles resulting in a high probability of voids or pinholes in the PTFE insulation. Another problem associated with ram extrusion is that it is not a continuous process so that the length of conductor (wire) being coated is limited to the size of the PTFE slug being ram extruded.
PTFE tape is skived from ram extruded cylinders and thus may also contain voids and pinholes arising from the ram extrusion process as well as being difficult to fold about small diameter conductors. As in the case of ram extruded PTFE, the tape length is also limited due to its having been skived from a cylinder of fixed diameter and length.
Although fluorocarbon materials that are melt extruded (thermoplastic) such as ethylene fluorinated ethylene propylene (ETFE) or fluorinated ethylene propylene (FEP) do not have the problems of ram extrusion heretofore described for PTFE, they are characterized by having lower temperature resistance than PTFE. For example, the temperature resistance of FEP is 400.degree. F. rather than the 500.degree. F. associated with PTFE.
As described in U.S. Pat. No. 4,711,833, it is known to deposit powdered polymers (including FEP and PTFE) electrostatically onto a collapsible mandrel in a process for making a seamless (endless) belt.
It has not been known heretofore to electrostatically deposit a fluorocarbon layer onto a fiberglass layer that encompasses an elongate electrical conductor (wire).
The present invention overcomes the processing limitations of PTFE as well as accommodates other fluorocarbon materials that can be rendered into a powder in preparation for electrostatic deposition without the need for solvents or the length and porosity problems associated with both ram extruded billets and skived tape.